The ever-increasing demand for portable electronic devices motivates technological improvements in energy conversion and storage units used in these devices. In developing the energy conversion and storage units (examples of which include batteries, capacitors and hybrid battery-capacitors), lightweight construction, long lifetime, high energy density, high power density and flexibility to meet various design and power consumption needs are important factors to consider. High power density is needed in providing fast bursts of current in power demanding applications such as camera flashlights, hard disk drives, high-resolution displays, etc. Electrochemical double layer capacitors (EDLCs), also known as supercapacitors, are very attractive power sources for those energy-hungry electronic devices. Supercapacitors are generally lightweight, having high power density and long durability. Since conventional batteries cannot provide for peaks of power required for high power density applications without getting too big in size, having a supercapacitor working alongside a battery in an electronic device would be an ideal solution. The supercapacitor-battery combination can provide the power boost the device needs while keeping battery dimensions small and extending battery lifetime.
Capacitor electrodes are commonly fabricated using various forms of carbon. Carbon has relatively high conductivity, light weight and high chemical stability under acidic and alkaline conditions, making it an ideal candidate for storing electrical charges. In supercapacitors, the surface area of the electrodes is of critical importance. Porous carbon structures having a high surface area, such as activated carbon, are commonly used. Recently, for further enhancing the surface area, various forms of nanostructured carbon are being used.
Future portable device concepts envisage devices that are flexible and/or transparent. One notable example is the so-called “Morph” concept developed by Nokia Corporation. In this concept, portable electrical devices are envisioned to be flexible and transparent, so that they blend more seamlessly with the way people live. Transparent and flexible electronic devices offer an entirely new aesthetic dimension. Furthermore, solar absorption capability may be built in, that charges the device, whilst batteries become smaller, longer lasting and faster to charge. The integrated electronics shown in the Morph concept could cost less and include more functionality in a much smaller space, even as interfaces are simplified and usability is enhanced. All of these new capabilities will unleash new applications and services that will allow people to communicate and interact in unprecedented ways.
An integral but yet unexplored part of such a flexible device is transparent power storage units. Such power storage units should meet both the power requirements and design requirements of the portable electronic devices. Also, it is very important that the flexible and transparent power storage units are made with simple and low cost processes.